The use of stent medical devices, or other types of endoluminal mechanical support devices, to keep a duct, vessel or other body lumen open in the human body has developed into a primary therapy for lumen stenosis or obstruction. The use of stents in various surgical procedures has quickly become accepted as experience with stent devices accumulates, and the number of surgical procedures employing them increases as their advantages become more widely recognized. For example, it is known to use stents in body lumens in order to maintain open passageways such as the prostatic urethra, the esophagus, the biliary tract, intestines, and various coronary arteries and veins, as well as more remote cardiovascular vessels such as the femoral artery, etc. There are two types of stents that are presently utilized: permanent stents and temporary stents. A permanent stent is designed to be maintained in a body lumen for an indeterminate amount of time. Temporary stents are designed to be maintained in a body lumen for a limited period of time in order to maintain the patency of the body lumen, for example, after trauma to a lumen caused by a surgical procedure or an injury. Permanent stents are typically designed to provide long-term support for damaged or traumatized wall tissues of the lumen. There are numerous conventional applications for permanent stents including cardiovascular, urological, gastrointestinal, and gynecological applications.
It is known that permanent stents, over time, become encapsulated and covered with endothelium tissues, for example, in cardiovascular applications. Similarly, permanent stents are known to become covered by epithelium, for example, in urethral applications.
Temporary stents, on the other hand are designed to maintain the passageway of a lumen open for a specific, limited period of time, and preferably do not become incorporated into the walls of the lumen by tissue ingrowth or encapsulation. Temporary stents may advantageously be eliminated from body lumens after a predetermined, clinically appropriate period of time, for example, after the traumatized tissues of the lumen have healed and a stent is no longer needed to maintain the patency of the lumen. For example, temporary stents can be used as substitutes for in-dwelling catheters for applications in the treatment of prostatic obstruction or other urethral stricture diseases. Another indication for temporary stents in a body lumen is after energy ablation, such as laser or thermal ablation, or irradiation of prostatic tissue, in order to control post-operative acute urinary retention or other body fluid retention.
It is known in the art to make both permanent and temporary stents from various conventional, biocompatible metals. However, there are several disadvantages that may be associated with the use of metal stents. For example, it is known that the metal stents may become encrusted, encapsulated, epithelialized or ingrown with body tissue. The stents are known to migrate on occasion from their initial insertion location. Such stents are known to cause irritation to the surrounding tissues in a lumen. Also, since metals are typically much harder and stiffer than the surrounding tissues in a lumen, this may result in an anatomical or physiological mismatch, thereby damaging tissue or eliciting unwanted biologic responses. Although permanent metal stents are designed to be implanted for an indefinite period of time, it is sometimes necessary to remove permanent metal stents. For example, if there is a biological response requiring surgical intervention, often the stent must be removed through a secondary procedure. If the metal stent is a temporary stent, it will also have to be removed after a clinically appropriate period of time. Regardless of whether the metal stent is categorized as permanent or temporary, if the stent has been encapsulated, epithelialized, etc., the surgical removal of the stent will resultingly cause undesirable pain and discomfort to the patient and possibly additional trauma to the lumen tissue. In addition to the pain and discomfort, the patient must be subjected to an additional time consuming and complicated surgical procedure with the attendant risks of surgery, in order to remove the metal stent.
Similar complications and problems, as in the case of metal stents, may well result when using permanent stents made from non-absorbable biocompatible polymer or polymer-composites although these materials may offer certain benefits such as reduction in stiffness.
It is known to use bioabsorbable and biodegradable materials for manufacturing temporary stents. The conventional bioabsorbable or bioresorbable materials from which such stents are made are selected to absorb or degrade over time, thereby eliminating the need for subsequent surgical procedures to remove the stent from the body lumen. In addition to the advantages attendant with not having to surgically remove such stents, it is known that bioabsorbable and biodegradable materials tend to have excellent biocompatibility characteristics, especially in comparison to most conventionally used biocompatible metals. Another advantage of stents made from bioabsorbable and biodegradable materials is that the mechanical properties can be designed to substantially eliminate or reduce the stiffness and hardness that is often associated with metal stents, which can contribute to the propensity of a stent to damage a vessel or lumen.
However, there are disadvantages known to be associated with the use of bioabsorbable or biodegradable stents. The disadvantages arise from the limitation of the material from which the stent is made. One of the problems associated with the current stents is that the materials break down too quickly. This improper breakdown or degradation of a stent into large, rigid fragments in the interior of a lumen, such as the urethra, may cause obstruction to normal flow, such as voiding, thereby interfering with the primary purpose of the stent in providing lumen patency. Alternatively, they take a long time to breakdown and stay in the target lumen for a considerable period of time after their therapeutic use has been accomplished. There is thus a long-term risk associated with these materials to form stones when implanted in a urine environment, for example, the urethra.
Accordingly, there is a need in this art for novel, temporary stents made from biodegradable polymers, wherein the stents remain functional in a body lumen for the duration of a prescribed, clinically appropriate period of time to accomplish the predetermined therapeutic purpose, and, then degrade without breaking down into large, rigid fragments, which may cause irritation, obstruction, pain or discomfort to the patient.
In a preferred embodiment of the present invention, the temporary stent readily passes out of the body as very soft particles or soft fibrous element or elements, and irritation, obstruction, pain or discomfort to the patient is either eliminated, or if present, is minimal.